Co3Gd4 Cage as Magnetic Refrigerant and Co3Dy3 Cage Showing Slow Relaxation of Magnetisation

Two structurally dissimilar 3d-4f cages having the formulae [(CoIII)3Gd4(μ3-OH)2(CO3) (O2CtBu)11(teaH)3]·5H2O (1) and [(CoIII)3Dy3(μ3-OH)4(O2CtBu)6(teaH)3]·(NO3)2·H2O (2) have been isolated under similar reaction conditions and stoichiometry of the reactants. The most important factor for structural diversity seems to be the incorporation of one μ3-carbonate anion in 1 and not in 2. Co atoms are in a +3 oxidation state in both complexes, as shown by the Bond Valence Sum (BVS) calculations and bond lengths, and as further supported by magnetic measurements. Co3Gd4 displays a significant magnetocaloric effect (−∆Sm = 25.67 J kg−1 K−1), and Co3Dy3 shows a single molecule magnet (SMM) behavior.

Alkoxo ligands such as N-substituted diethanolamines have been widely used in the synthesis of 3d-4f cages with different numbers of metal centers and magnetic properties [49][50][51][52]. Recently, we have reported a series of complexes based on the N-nbutyldiethanolamine ligand [53]. With the intention of obtaining new heterometallic cages with an increased magnetic density by decreasing the content of non-metallic elements, we used an alkoxo ligand containing more hydroxyl groups (triethanolamine).

Synthesis and Structural Analysis
In this work, we have employed triethanolamine (teaH 3 ) with the aim of obtaining new heterometallic cages. The use of the said ligand for such systems is rare [54][55][56][57]. We have reacted it with a small dimer, [Co 2 (µ-OH 2 ) (O 2 C t Bu) 4 ]·(HO 2 C t Bu) 4 (hereafter: Co 2 ), and lanthanide salts, and have successfully isolated two complexes with different structural features under similar reaction conditions and stoichiometry of the reactants. The reaction of Co 2 and teaH 3 with Gd(NO 3 ) 3 ·6H 2 O in a 1:1:1 molar ratio in CH 3 CN gave the compound [(Co III ) 3 Gd 4 (µ 3 -OH) 2 (CO 3 ) (O 2 C t Bu) 11 X-ray crystallography reveals that complex 1 crystallises in the P-1 space group and is a heterometallic heptanuclear cage primarily composed of three cobalt centres and four Gd III ions ( Figure 1). All three cobalt centres are in a +3 oxidation state, as shown by the Bond valence sum (BVS) calculations and bond lengths [58]. Two µ 3 -hydroxo groups and oxygen atoms of one carbonate anion interconnect the metal centres of this heptanuclear core ( Figure 2). Peripheral ligation is provided by three doubly deprotonated triethanolamine ligands (teaH). One can observe that N atoms coordinate to the cobalt ions and that oxygens coordinate to the Gd III ions. The central portion is also enveloped by a hydrophobic covering of eleven pivalate ligands bridging in the 2.11 mode. Therefore, all the cobalt ions end up with an octahedral geometry (with O5N coordination), and the Gd III ions feature a distorted square antiprismatic geometry. The average Co III −O and Co III −N bond lengths are 1.90 (5) and 1.98 (6) Å, respectively, and the average Gd−O bond length is 2.37 (5) Å.
cages with an increased magnetic density by decreasing the content of non ments, we used an alkoxo ligand containing more hydroxyl groups (trietha
X-ray crystallography reveals that complex 1 crystallises in the P-1 spa is a heterometallic heptanuclear cage primarily composed of three cobalt cen Gd III ions ( Figure 1). All three cobalt centres are in a +3 oxidation state, as Bond valence sum (BVS) calculations and bond lengths [58]. Two μ3-hydrox oxygen atoms of one carbonate anion interconnect the metal centres of this core ( Figure 2). Peripheral ligation is provided by three doubly deprotona lamine ligands (teaH). One can observe that N atoms coordinate to the co that oxygens coordinate to the Gd III ions. The central portion is also envelo drophobic covering of eleven pivalate ligands bridging in the 2.11 mode. the cobalt ions end up with an octahedral geometry (with O5N coordina Gd III ions feature a distorted square antiprismatic geometry. The averag Co III −N bond lengths are 1.90 (5) and 1.98 (6) Å , respectively, and the averag length is 2.37 (5) Å .  Complex 2 crystallises in the monoclinic space group P21/c and is another example of a mixed metal system comprising three Co III and three Dy III ions ( Figure 3). The metal centres and oxygen atoms in the central hexanuclear unit are interlinked in a hemicubane-like fashion by four μ3-hydroxo groups ( Figure 4). Three doubly deprotonated triethanolamine ligands (teaH) are also part of this structural aggregation coordinating via the N atom to the Co III ions and then bridging the Co III centres to the Dy III ions via their two μ2-alkoxo groups. Six pivalate groups bridging in the 2.11 mode and three water molecules, one each coordinating to the Dy III ions, also surround the basic unit. With all these coordinating atoms of the ligands, the Co III ions end up being six-coordinated with an octahedral geometry having average Co−O and Co−N bond distances of 1.90 (1) and 1.97 (2) Å , respectively. All the Dy III ions are eight-coordinated having a distorted square antiprismatic geometry and average Dy−O bond length of 2.36  Complex 2 crystallises in the monoclinic space group P21/c and is another example of a mixed metal system comprising three Co III and three Dy III ions ( Figure 3).  The metal centres and oxygen atoms in the central hexanuclear unit are i in a hemicubane-like fashion by four μ3-hydroxo groups ( Figure 4). Thr deprotonated triethanolamine ligands (teaH) are also part of this structural ag coordinating via the N atom to the Co III ions and then bridging the Co III cen Dy III ions via their two μ2-alkoxo groups. Six pivalate groups bridging in the and three water molecules, one each coordinating to the Dy III ions, also sur basic unit. With all these coordinating atoms of the ligands, the Co III ions end six-coordinated with an octahedral geometry having average Co−O and C distances of 1.90 (1) and 1.97 (2) Å , respectively. All the Dy III ions are eight-co having a distorted square antiprismatic geometry and average Dy−O bond len  The metal centres and oxygen atoms in the central hexanuclear unit are interlinked in a hemicubane-like fashion by four µ 3 -hydroxo groups ( Figure 4). Three doubly deprotonated triethanolamine ligands (teaH) are also part of this structural aggregation, coordinating via the N atom to the Co III ions and then bridging the Co III centres to the Dy III ions via their two µ 2 -alkoxo groups. Six pivalate groups bridging in the 2.11 mode and three water molecules, one each coordinating to the Dy III ions, also surround the basic unit. With all these coordinating atoms of the ligands, the Co III ions end up being six-coordinated with an octahedral geometry having average Co−O and Co−N bond distances of 1.90 (1) and 1.97 (2)   The careful study of the two structures (1 and 2) shows some interesting features. Although the number of chelating teaH ligands is the same in both complexes, the number of pivalate groups is reduced to almost half from the former to the latter complex (from 11 in 1 to 6 in 2). The incorporation of one μ3-carbonate anion seems the most important factor for this structural diversity. The number of μ3-hydroxo groups is also doubled from two to four from the former to the latter complex, respectively. One μ3-hydroxo group is found bridging the three Dy III centres in complex 2 and is not present in comple 1. Another difference is that each of the Dy III centres in the latter complex is coordinated to one water molecule to complete the coordination sphere.

Magnetic Studies
Polycrystalline samples of 1 & 2 were used to collect the dc susceptibility data in th temperature range of 1.  The careful study of the two structures (1 and 2) shows some interesting features. Although the number of chelating teaH ligands is the same in both complexes, the number of pivalate groups is reduced to almost half from the former to the latter complex (from 11 in 1 to 6 in 2). The incorporation of one µ 3 -carbonate anion seems the most important factor for this structural diversity. The number of µ 3 -hydroxo groups is also doubled from two to four from the former to the latter complex, respectively. One µ 3 -hydroxo group is found bridging the three Dy III centres in complex 2 and is not present in complex 1. Another difference is that each of the Dy III centres in the latter complex is coordinated to one water molecule to complete the coordination sphere.

Magnetic Studies
Polycrystalline samples of 1 & 2 were used to collect the dc susceptibility data in the temperature range of 1.8-300 K at 0.1 T. The DC magnetic studies ( Figure 5) reveal room temperature χ M T values of 30.56 and 42.41 cm 3 mol −1 K for 1 and 2, respectively, which are quite close to the estimated values of 29.24 (four uncoupled Gd III , g = 1.99) and 42.71 (2b, three uncoupled Dy III , g = 4/3). Upon lowering the temperature, the χ M T products stay nearly constant for complex 1 up to 40 K, where an abrupt decrease is witnessed, reaching a value of 22.89 cm 3 mol −1 K at 0.1 T and 1.8 K. This behaviour can be ascribed to the isotropic nature of the Gd III ions. For complex 2, upon lowering the temperature, the χ M T values are nearly constant up to 60 K, followed by an abrupt decrease, reaching a value of 15.75 cm 3 mol −1 K at 0.1 T and 1.8 K. This fall could be due to the depopulation of the Stark (m J ) sublevels of the ground J multiplet, with the likelihood of a feeble antiferromagnetic exchange and dipolar interactions also backing the behaviour.
For complex 1, the field dependence of magnetisation shows a saturation value of 28.8 Nµ B at 7 T ( Figure S1). This is compatible with the predicted value of 28 Nµ B . The entropy variations (∆S m ) for 1 were estimated using the Maxwell equation ∆S m (T) ∆H = [∂M(T,H)/∂T] H dH [59]. The −∆S m vs. T plot gradually increases from 9 K to 2 K (Figure 6), reaching a maximum of 25.67 J kg −1 K −1 at 3 K and 7 T. These results compare well with the other Co−Gd cages in the literature [60][61][62][63].
The M/Nµ B vs. H plot for 2 ( Figure S2) shows an abrupt increase with the increasing field reaching a value of 17.28 Nµ B but not saturating, even at 7 T. This is usually due to the presence of anisotropy and significant crystal field effects from the Dy III ions [54][55][56][57]. The non-superposition of the M/Nµ B versus H/T plot of complex 2 ( Figure S3) confirms the presence of significant anisotropy in the molecule. For complex 1, the field dependence of magnetisation shows a saturation 28.8 NμB at 7 T ( Figure S1). This is compatible with the predicted value of 28 N entropy variations (∆Sm) for 1 were estimated using the Maxwell equation ∆S ∫[∂M(T,H)/∂T]HdH [59]. The −∆Sm vs. T plot gradually increases from 9 K to 2 K (F reaching a maximum of 25.67 J kg −1 K −1 at 3 K and 7 T. These results compare w the other Co−Gd cages in the literature [60][61][62][63].   For complex 1, the field dependence of magnetisation shows a saturation v 28.8 NμB at 7 T ( Figure S1). This is compatible with the predicted value of 28 N entropy variations (∆Sm) for 1 were estimated using the Maxwell equation ∆Sm ∫[∂M(T,H)/∂T]HdH [59]. The −∆Sm vs. T plot gradually increases from 9 K to 2 K (Fi reaching a maximum of 25.67 J kg −1 K −1 at 3 K and 7 T. These results compare w the other Co−Gd cages in the literature [60][61][62][63].  Figure S2) shows an abrupt increase with the inc field reaching a value of 17.28 NμB but not saturating, even at 7 T. This is usually the presence of anisotropy and significant crystal field effects from the Dy III ions The non-superposition of the M/NμB versus H/T plot of complex 2 ( Figure S3) c the presence of significant anisotropy in the molecule.
Alternating current susceptibility data for 2 was collected at a zero dc field u K at the 3.5 Oe ac field in the frequency range of 1-800 Hz. Both the in-pha out-of-phase susceptibilities show temperature-dependent ac signals below 10 K ( S4 and S5), indicating the slow relaxation of magnetisation. Due to quantum tunne the magnetisation (QTM), no full maxima were observed [64,65]. The data was Alternating current susceptibility data for 2 was collected at a zero dc field up to 1.8 K at the 3.5 Oe ac field in the frequency range of 1-800 Hz. Both the in-phase and out-of-phase susceptibilities show temperature-dependent ac signals below 10 K (Figures S4 and S5), indicating the slow relaxation of magnetisation. Due to quantum tunnelling of the magnetisation (QTM), no full maxima were observed [64,65]. The data was remeasured in the presence of an optimum static dc field of 2000 Oe to minimise the quantum tunnelling. Peak maxima were observed under this field below 5 K in the out-of-phase (χ") vs. T plot (Figure 7 (left)), confirming the field-induced SMM behaviour [54][55][56][57]. The frequencydependent in-phase (χ ) and out-of-phase (χ") susceptibility plots also confirmed this behaviour ( Figure S7 and Figure 7 (right)). The magnetic properties of this compound strongly resemble one of our previously reported compounds because of the similar core structure [53]. ured in the presence of an optimum static dc field of 2000 Oe to minimise the quantum tunnelling. Peak maxima were observed under this field below 5 K in the out-of-phase ('') vs. T plot (Figure 7 (left)), confirming the field-induced SMM behaviour [54][55][56][57]. The frequency-dependent in-phase (') and out-of-phase ('') susceptibility plots also confirmed this behaviour ( Figures S7 and 7 (right)). The magnetic properties of this compound strongly resemble one of our previously reported compounds because of the similar core structure [53]. The best-fitting results for the Arrhenius equation (Equation (1)) [66,67] gave an energy barrier Ueff ≈ 17.5 K and a relaxation time τ0 ≈ 2.3×10 −6 s from the frequency dependencies of the ac susceptibility ( Figure 8).
where k is the Boltzmann constant, and 1/τ0 is the pre-exponential factor.
The Cole−Cole plot ('' vs. ') is shown in the inset of Figure 8 as evidence of the relaxation process occurring in complex 2.

Materials and Methods
Both complexes were synthesised from the starting material [Co2(μ-OH2)(O2C t Bu)4] · (HO2C t Bu)4, Co2. All the reagents were used as received from Sigma Aldrich without any The best-fitting results for the Arrhenius equation (Equation (1)) [66,67] gave an energy barrier U eff ≈ 17.5 K and a relaxation time τ 0 ≈ 2.3 × 10 −6 s from the frequency dependencies of the ac susceptibility ( Figure 8).
where k is the Boltzmann constant, and 1/τ 0 is the pre-exponential factor. ured in the presence of an optimum static dc field of 2000 Oe to minimise the quantum tunnelling. Peak maxima were observed under this field below 5 K in the out-of-phase ('') vs. T plot (Figure 7 (left)), confirming the field-induced SMM behaviour [54][55][56][57]. The frequency-dependent in-phase (') and out-of-phase ('') susceptibility plots also confirmed this behaviour ( Figures S7 and 7 (right)). The magnetic properties of this compound strongly resemble one of our previously reported compounds because of the similar core structure [53]. The best-fitting results for the Arrhenius equation (Equation (1)) [66,67] gave an energy barrier Ueff ≈ 17.5 K and a relaxation time τ0 ≈ 2.3×10 −6 s from the frequency dependencies of the ac susceptibility ( Figure 8).
where k is the Boltzmann constant, and 1/τ0 is the pre-exponential factor.
The Cole−Cole plot ('' vs. ') is shown in the inset of Figure 8 as evidence of the relaxation process occurring in complex 2.

Materials and Methods
Both complexes were synthesised from the starting material [Co2(μ-OH2)(O2C t Bu)4] · (HO2C t Bu)4, Co2. All the reagents were used as received from Sigma Aldrich without any The Cole−Cole plot (χ" vs. χ ) is shown in the inset of Figure 8 as evidence of the relaxation process occurring in complex 2.

Materials and Methods
Both complexes were synthesised from the starting material [Co 2 (µ-OH 2 )(O 2 C t Bu) 4 ] ·(HO 2 C t Bu) 4 , Co 2 . All the reagents were used as received from Sigma Aldrich without any further purification. The magnetic behaviour of the compounds was studied on a Quantum Design SQUID-VSM magnetometer. Diamagnetic corrections were made with Pascal's constants for all of the constituent atoms [68]. Magnetic susceptibility measurements were performed in 1.8-300 K with an applied field of 0.1 T. Infrared spectra were collected for the solid samples using KBr pellets on a Perkin Elmer Fourier-transform infrared (FTIR) spectrometer in the range of 400-4000 cm −1 . An Elementar vario Microcube elemental analyser was used to get the elemental analysis data.
Single-crystal X-ray structural studies of 1 & 2 were carried out on a CCD Bruker SMART APEX 2 CCD diffractometer under the cold flow of an Oxford device. Data were collected using graphite−monochromated Mo Kα radiation (λα = 0.71073 Å). Structure solution, refinement and data reduction were carried out by (SHELXTL-97), SAINT and SAD-ABS programs [69][70][71]. Large solvent accessible voids are present in the structures, which are probably filled with disordered solvent molecules. Therefore, SQUEEZE/PLATON was used to remove or fix these disorders [72]. The CIF format of the data is available in CCDC numbers 1,050,639 and 1,050,640 and is also summarized in Table S2.

Conclusions
Two structurally dissimilar heterometallic aggregates were successfully synthesised from a preformed precursor and triethanolamine. The Gd analogue displays a significant magnetocaloric effect, and the Dy-containing compound shows the slow relaxation of the magnetisation. The results are a good addition to the 3d-4f heterometallic aggregates in general and those obtained from polyalcohol-based ligands in particular. This work should be useful to the sensible strategy and production of a library of heterometallic magnetic materials employing different polytopic ligands.
Supplementary Materials: The following supporting information can be downloaded online, Figure S1: Field dependencies of isothermal normalised magnetisations for complex 1 collected for temperatures ranging from 2-10 K. Figure S2: Field dependencies of isothermal normalised magnetisations for complex 2 collected for temperatures ranging from 2-10 K. Figure S3: M/Nµ B vs. H/T plots for complex 2 at 2-10 K. Figure S4: Temperature dependence of the in-phase (χ ) ac susceptibility for complex 2 under a zero dc field. Figure S5: Temperature dependence of the out-of-phase (χ") ac susceptibility for complex 2 under a zero dc field. Figure S6: Temperature dependence of the in-phase (χ ) ac susceptibility for complex 2 under a 2000 Oe dc field. Figure S7: Frequency dependence of the in-phase (χ ) ac susceptibility for complex 2 under a 2000 Oe dc field. Table S1: BVS calculations for complexes 1 and 2. Table S2: Crystal data and structure refinement for complexes 1 and 2. Data Availability Statement: This study did not report any data.